U.S. patent number 8,564,916 [Application Number 13/028,152] was granted by the patent office on 2013-10-22 for photovoltaic array ground fault detection method for utility-scale grounded solar electric power generating systems.
This patent grant is currently assigned to Western Gas and Electric Company. The grantee listed for this patent is Brian Hinman, Hossein Kazemi, Viggo Selchau-Hansen. Invention is credited to Brian Hinman, Hossein Kazemi, Viggo Selchau-Hansen.
United States Patent |
8,564,916 |
Kazemi , et al. |
October 22, 2013 |
Photovoltaic array ground fault detection method for utility-scale
grounded solar electric power generating systems
Abstract
Various methods and apparatus are described for a photovoltaic
system. In an embodiment, a hybrid grounding circuit as well as a
ground fault monitoring circuit are in the inverter circuitry with
its switching devices that generate three-phase Alternating Current
(AC) voltage. The three-phase AC voltage is supplied to a utility
power grid interface transformer, where a primary-side common node
of the Utility Power grid interface transformer is connected to
Earth ground. Each inverter has 1) its own set of isolation
contacts to connect as well as isolate this particular inverter
from the utility grid interface transformer, and 2) control
components in the ground fault monitoring circuit for controlling
operation of the isolation contacts based off a presence of the
ground fault detected by the ground fault monitor circuit for that
inverter. The inverter circuit receives a DC voltage supplied from
its own set of ungrounded Concentrated PhotoVoltaic modules.
Inventors: |
Kazemi; Hossein (San Francisco,
CA), Selchau-Hansen; Viggo (Dover, MA), Hinman; Brian
(Los Gatos, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Kazemi; Hossein
Selchau-Hansen; Viggo
Hinman; Brian |
San Francisco
Dover
Los Gatos |
CA
MA
CA |
US
US
US |
|
|
Assignee: |
Western Gas and Electric
Company (Thousand Oaks, CA)
|
Family
ID: |
44369496 |
Appl.
No.: |
13/028,152 |
Filed: |
February 15, 2011 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20110199707 A1 |
Aug 18, 2011 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61338313 |
Feb 16, 2010 |
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61424537 |
Dec 17, 2010 |
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61370001 |
Aug 2, 2010 |
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61343070 |
Apr 23, 2010 |
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Current U.S.
Class: |
361/47 |
Current CPC
Class: |
H02H
7/20 (20130101); H02H 7/1222 (20130101); H02H
3/33 (20130101); Y02E 10/56 (20130101) |
Current International
Class: |
H02H
3/16 (20060101) |
Field of
Search: |
;361/42,47 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Wiles, John, "Photovoltaic Power Systems and the 2005 National
Electrical Code: Suggested Practices", Oct. 4, 2007. cited by
examiner.
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Primary Examiner: Fureman; Jared
Assistant Examiner: Comber; Kevin J
Attorney, Agent or Firm: Rutan & Tucker, LLP
Parent Case Text
RELATED APPLICATIONS
This application is a continuation in part of the following and
claims the benefit of and priority under 35 USC 119(e) to U.S.
Provisional Application titled "SINGLE STAGE DC-TO-AC CONVERSION
FOR HIGH EFFICIENCY OPERATION OF CONCENTRATING PHOTOVOLTAIC
SYSTEMS" filed on Feb. 16, 2010 having application Ser. No.
61/338,313 and U.S. Provisional Application titled "INTEGRATED
ELECTRONICS SYSTEM" filed on Dec. 17, 2010 having application Ser.
No. 61/424,537, and U.S. Provisional Application titled
"PHOTOVOLTAIC ARRAY GROUND FAULT DETECTION METHOD FOR UTILITY-SCALE
GROUNDED SOLAR ELECTRIC POWER GENERATING SYSTEMS" filed on Aug. 2,
2010 having application Ser. No. 61/370,001, and U.S. Provisional
Application titled "SOLAR CELL SUBSTRING GROUNDING TO MANAGE
INVERTER INPUT VOLTAGE" filed on Apr. 23, 2010 having application
Ser. No. 61/343,070.
Claims
We claim:
1. An apparatus for a photovoltaic system, comprising: a hybrid
grounding circuit as well as a ground fault monitoring circuit in
inverter circuitry with switching devices that are configured to
generate three-phase Alternating Current (AC) voltage supplied to a
utility power grid interface transformer, where a primary-side
common node of the utility power grid interface transformer is
connected to Earth ground, where the inverter circuitry includes at
least a first inverter circuit and a second inverter circuit, where
each inverter circuit has 1) its own set of isolation contacts to
connect as well as isolate this particular inverter circuit from
the utility grid interface transformer, and 2) control components
in the ground fault monitoring circuit for controlling operation of
the isolation contacts based off a presence of the ground fault
detected by the ground fault monitor circuit for that inverter
circuit, where the first inverter circuit receives a first DC
voltage supplied from its own set of ungrounded PhotoVoltaic (PV)
modules, where the second inverter circuit receives a second DC
voltage supplied from its own set of ungrounded PV modules, where
multiple solar arrays, each with their one or more inverter
circuits, directly couple their three phase AC output to the same
utility power grid interface transformer, where each inverter
circuit in the grounded system has its own ground fault monitoring
circuit, where the ground fault monitoring circuit detects the
presence of the ground fault via a change in 1) voltage, 2) current
or 3) both and the detection method occurs via a differential sense
method, where in the hybrid grounding circuit when in operation
producing AC, each inverter circuit gets referenced to 1) the
primary-side common node of the utility power grid interface
transformer which is connected to Earth ground during the operation
of that inverter circuit and 2) referenced to Earth ground by a
grounding switching device in an input circuit of that inverter
circuit prior to the switching devices of the inverter circuit
producing three phase AC power, and where the ground fault
monitoring circuit is configured to detect PV array ground leakage
before the isolation contacts of the first inverter circuit connect
to the utility power grid interface transformer, which prevents
disconnecting all of the inverter circuits in the entire system due
to a ground fault on a single set of PV modules.
2. The apparatus for a photovoltaic system of claim 1, further
comprising: where the ground fault monitoring circuit is configured
to detect the presence of the ground fault in the ungrounded PV
modules that supply Direct Current (DC) power to grounded inverter
circuit, where the first and second inverter circuits use Space
Vector Modulated bridge switches, generating a nominal 480 V AC
three phase power, that directly couple to the utility power grid
transformer, without connection through an isolation transformer
and then to the utility grid transformer, in where a neutral wire
of the primary side of the utility power grid interface transformer
is referenced to Earth ground.
3. The apparatus for a photovoltaic system of claim 1, where the
isolation contacts and control components of the ground fault
monitoring circuit are configured to prevent the 1) disconnection
of an entire solar power generating system or 2) disconnection of
an inverter group from the utility power grid interface transformer
due to the ground fault occurring in an individual inverter circuit
or its associated PV modules, by a localization of the ground fault
to 1) a specific inverter circuit from an inverter group coupling
to the utility grid power transformer or 2) even more specifically,
a specific set of PV modules feeding a specific inverter circuit,
which also reduce corrective maintenance costs.
4. The apparatus for a photovoltaic system of claim 1, where the
inverter circuitry receives a bipolar DC voltage supplied from its
own set of PV modules, where a switching device in the input
circuit of each inverter circuit is used to create a common
reference point for the positive VDC and the negative VDC inputs
from the bipolar DC voltage of the PV array, and where the ground
fault monitoring circuit localizes of the ground fault to a
specific set of PV modules feeding a specific inverter circuit by
when the detected ground fault voltage has a negative voltage
component, then the ground fault is coming from the set of PV
modules supplying the negative DC voltage; and likewise when the
detected ground fault voltage has a positive voltage component,
then the ground fault is coming from the set of PV modules
supplying the positive DC voltage.
5. The apparatus for a photovoltaic system of claim 1, where the
first inverter circuit receives a bipolar DC voltage supplied from
its own set of PV modules, and where the hybrid grounding circuit
includes an input DC grounding circuit located in each inverter
circuit that electrically couples to 1) a string of PV cells from a
PV array and 2) the primary-side common node of the utility power
grid interface transformer is connected to the Earth ground, where
electrical components in the input DC grounding circuit include 1)
one or more switching devices, one or more load resistors, and one
or more over current devices to cause the DC power from that string
of PV cells to be connected to the Earth ground when the first
inverter circuit is not generating the three phase AC power out,
and the electrical components in the input DC grounding circuit
also includes 1) a contact 2) switch or 3) both to create a dynamic
common zero Volts DC reference point for the bipolar DC voltage
supplied to that inverter from its set of PV modules.
6. The apparatus for a photovoltaic system of claim 1, where the
ground fault monitoring circuit contains a DC imbalance sensing
circuit to assist in fault protection in determining which set of
PV modules supplying power to the common utility grid transformer
has a ground fault both 1) prior to inverter start up and 2) during
the inverter circuit operation while producing the three phase AC
voltage.
7. The apparatus for a photovoltaic system of claim 1, where the
hybrid grounding circuit in the first inverter circuit uses relays
to maintain its own set of ungrounded PV modules at a safe voltage
while the first inverter circuit is disconnected from the utility
grid interface transformer by connecting the first DC voltage
supplied from the set of ungrounded PV modules feeding the first
inverter circuit through a switching device to the Earth
ground.
8. The apparatus for a photovoltaic system of claim 7, where the
hybrid grounding circuit in the first inverter circuit couples the
set of PV modules to an Earth Ground in the input circuit of the
first inverter circuit through the switching device, a load
resistor, and an over current device, and where the switching
device may actuate, A) by electrically opening a contact or switch,
B) electrically by closing a contact or switch, C) by the switching
device starting to conduct, and D) by any combination of the three,
to create an electrical path.
9. The apparatus for a photovoltaic system of claim 7, where the
hybrid grounding circuit in the first inverter circuit has a DC
common reference point control relay that actuates one or more
contacts to create a common DC reference point for a bipolar DC
voltage input from the ungrounded PV modules while the first
inverter circuit is turned off, and also while an inverter logic is
powered but the first inverter circuit is disconnected from the
utility power grid and not operating, and when in these conditions,
the inverter bridge switches are open, resulting in an electrical
open circuit at each PV array pole, and consequently, no current
flows in the PV solar array even if well-illuminated by sunlight
because a complete ground path cannot be established, and where an
input of the ground fault monitoring circuit of each inverter
circuit is equipped with a residual current monitor (RCM).
10. The apparatus for a photovoltaic system of claim 9, where the
ground fault monitoring circuit detects when a ground leak occurs
in the PV modules, a ground current will flow through the PV
modules, ground fault resistance, a load resistor, a fuse, and the
switching device completing the ground path between the normally
ungrounded PV modules and Earth ground, and with the ground fault
current flowing, the ground fault monitoring circuit can detect the
presence of the ground fault on the ungrounded PV array, and in
addition, when the fault current exceeds the fuse rating, then the
fuse will open and interrupt the fault current; however, the
voltage from the fault will now be sensed on an input legs of a
voltage differential sense circuit.
11. The apparatus for a photovoltaic system of claim 10, where the
ground fault monitoring circuit detects when the first inverter
circuit is scheduled to put be on-line, the inverter controller is
configured to interrogate the open fuse sense lines and when the
inverter controller senses any voltage between the two open fuse
sense lines, a fuse has opened, indicating a ground fault in the PV
solar arrays, and the inverter controller keeps the inverter
circuit off-line by keeping the isolation contacts open.
12. An apparatus for a photovoltaic system, comprising: a hybrid
grounding circuit as well as a ground fault monitoring circuit in
inverter circuitry with switching devices that are configured to
generate three-phase Alternating Current (AC) voltage supplied to a
utility power grid interface transformer, where a primary-side
common node of the utility power grid interface transformer is
connected to Earth ground, where each inverter has 1) its own set
of isolation contacts to connect as well as isolate this particular
inverter from the utility grid interface transformer, and 2)
control components in the ground fault monitoring circuit for
controlling operation of the isolation contacts based off a
presence of the ground fault detected by the ground fault monitor
circuit for that inverter, where a first inverter circuit receives
a DC voltage supplied from its own set of ungrounded PhotoVoltaic
(PV) modules, where the ground fault monitoring circuit is
configured when it detects no fuse failure, an inverter controller
then puts the first inverter circuit on-line with the utility power
grid by closing the isolation contacts, and also energizes a PV
array DC common reference point control relay to interconnect and
float DC voltage midpoints of a PV array, where a system ground is
now established by the primary-side common node of the utility
power grid interface transformer being connected to Earth ground,
and a residual current monitor of the ground fault monitor circuit
is now active and responsive to future PV array ground leaks, and
when a ground occurs in the PV array coupled to the first inverter
circuit, the ground fault is sensed by the residual current
monitor, then the monitor is configured to cause an opening of the
isolation contacts, and the PV array with the ground fault and the
first inverter circuit are then isolated from the utility grid
interface transformer but the transformer continues to receive AC
voltage from any other inverter circuits coupling to that
transformer and continues to produce power.
13. A method for grounding a photovoltaic system, comprising:
generating three-phase Alternating Current (AC) voltage supplied to
a utility power grid interface transformer with inverter circuitry
with switching devices, where a primary-side common node of the
utility power grid interface transformer is connected to Earth
ground, where the inverter circuitry includes at least a first
inverter and a second inverter; isolating each inverter with its
own set of isolation contacts from the utility grid interface
transformer; using control components in a ground fault monitoring
circuit for controlling operation of the isolation contacts based
off a presence of a ground fault detected by the ground fault
monitor circuit for that inverter, where the first inverter circuit
receives a first DC voltage supplied from its own set of ungrounded
PhotoVoltaic (PV) modules, where the second inverter circuit
receives a second DC voltage supplied from its own set of
ungrounded PV modules; detecting PV array ground leakage before the
isolation contacts of the first inverter circuit connect to the
utility power grid interface transformer, which prevents
disconnecting all of the inverter circuits in the entire system due
to a ground fault on a single set of modules; and causing the
ground fault monitoring circuit when it detects no fuse failure, to
put the first inverter circuit on-line with the utility power grid
by closing the isolation contacts, and also energizes a PV array DC
common reference point control relay to interconnect and float DC
voltage midpoints of a array, where a system ground is now
established by the primary-side common node of the utility power
grid interface transformer being connected to Earth ground, and a
residual current monitor of the ground fault monitor circuit is now
active and responsive to future array ground leaks, and when a
ground occurs in the array, the ground fault is sensed by the
residual current monitor, then the monitor is configured to cause
an opening of the isolation contacts, and the specific array with
the ground fault and its inverter circuit are then isolated from
the utility grid interface transformer but the transformer
continues to receive AC voltage from any other inverter circuits
coupling to that transformer and continues to produce power.
14. The method for the photovoltaic system of claim 13, further
comprising: using relays to maintain a set of PV modules from a PV
array at a safe voltage while the first inverter circuit is
disconnected from the utility grid interface transformer by
connecting the first DC output of the set of PV modules feeding the
first inverter circuit through a switching device to the Earth
ground.
15. The method for the photovoltaic system of claim 13, further
comprising: receiving a bipolar DC voltage supplied from its own
set of PV modules in the first inverter circuit; using an input DC
grounding circuit located in each inverter circuit that
electrically couples to 1) a string of PV cells from a PV array and
the primary-side common node of the utility power grid interface
transformer connected to the Earth ground; and causing the DC power
from that string of PV cells to be connected to the Earth ground
when the first inverter circuit is not generating the three phase
AC power out with electrical components in the input DC grounding
circuit selected from any of 1) one or more switching devices, one
or more load resistors, and one or more over current devices, and
the electrical components in the input DC grounding circuit also
includes 1) a contact 2) switch or 3) both to create a dynamic
common zero Volts DC reference point for the bipolar DC voltage
supplied to that inverter from its set of PV modules.
Description
NOTICE OF COPYRIGHT
A portion of the disclosure of this patent document contains
material that is subject to copyright protection. The copyright
owner has no objection to the facsimile reproduction by anyone of
the interconnect as it appears in the Patent and Trademark Office
patent file or records, but otherwise reserves all copyright rights
whatsoever.
FIELD
In general, a photovoltaic system having a hybrid grounding circuit
as well as a ground fault monitoring circuit is discussed.
BACKGROUND
Two methods have been used in the past to convert DC for a solar
array in AC voltage. A DC-DC boost converter can be used to
increase the string voltage enough for a sine-triangle PWM
inversion. Alternatively, a transformer can be used after the
inverter circuit to step up the inverter output. Either solution
adds cost and reduces efficiency and reliability.
SUMMARY
Various methods and apparatus are described for a photovoltaic
system. In an embodiment, a hybrid grounding circuit as well as a
ground fault monitoring circuit are in the inverter circuitry with
its switching devices that generate three-phase Alternating Current
(AC) voltage. The three-phase AC voltage is supplied to a utility
power grid interface transformer, where a primary-side common node
of the Utility Power grid interface transformer is connected to
Earth ground. Each inverter has 1) its own set of isolation
contacts to connect as well as isolate this particular inverter
from the utility grid interface transformer, and 2) control
components in the ground fault monitoring circuit for controlling
operation of the isolation contacts based off a presence of the
ground fault detected by the ground fault monitor circuit for that
inverter. The inverter circuit receives a DC voltage supplied from
its own set of ungrounded Concentrated PhotoVoltaic (CPV)
modules.
BRIEF DESCRIPTION OF THE DRAWINGS
The multiple drawings refer to the embodiments of the
invention.
FIG. 1 illustrates a diagram of an embodiment of an ungrounded
photovoltaic system feeding a hybrid grounding circuit as well as a
ground fault monitoring circuit for inverter circuitry with
switching devices that generate three-phase Alternating Current
(AC) voltage supplied to a utility power grid interface
transformer.
FIGS. 2a and 2b illustrate a diagram of an embodiment of a hybrid
grounding circuit and a ground fault monitoring circuit in the
inverter circuitry that generates AC voltage from the DC supplied
from the CPV solar array.
FIG. 3 illustrates a diagram of an embodiment of a voltage
differential sense circuit to sense an open fuse.
FIG. 4 illustrates a diagram of an embodiment of two inverters from
the same array having their own isolation contacts and supplying
power to a common grid transformer.
FIG. 5 illustrates a diagram of an embodiment of a method of solar
cell string voltage management, used in conjunction with a
single-stage SVM inverter.
FIG. 6 illustrates a diagram of an embodiment of a string of CPV
modules and their CPV cells supplying power to an inverter
circuit.
FIG. 7 illustrates a diagram of an embodiment of the physical and
electrical arrangement of modules in a representative tracker
unit.
FIG. 8 illustrates a table of an embodiment of available verses
required DC input voltage from a string of CPV modules.
While the invention is subject to various modifications and
alternative forms, specific embodiments thereof have been shown by
way of example in the drawings and will herein be described in
detail. The invention should be understood to not be limited to the
particular forms disclosed, but on the contrary, the intention is
to cover all modifications, equivalents, and alternatives falling
within the spirit and scope of the invention.
DETAILED DISCUSSION
In the following description, numerous specific details are set
forth, such as examples of specific voltages, named components,
connections, types of circuits, etc., in order to provide a
thorough understanding of the present invention. It will be
apparent, however, to one skilled in the art that the present
invention may be practiced without these specific details. In other
instances, well known components or methods have not been described
in detail but rather in a block diagram in order to avoid
unnecessarily obscuring the present invention. Further specific
numeric references such as a first inverter, may be made. However,
the specific numeric reference should not be interpreted as a
literal sequential order but rather interpreted that the first
inverter is different than a second inverter. Thus, the specific
details set forth are merely exemplary. The specific details may be
varied from and still be contemplated to be within the spirit and
scope of the present invention. The specific details may be varied
from and still be contemplated to be within the spirit and scope of
the present invention.
In general, various methods and apparatus associated with a hybrid
grounding circuit as well as a ground fault monitoring circuit for
a photovoltaic system are discussed. In an embodiment, the inverter
circuitry has a hybrid grounding circuit as well as a ground fault
monitoring circuit. The inverter circuitry also has switching
devices that generate three-phase AC voltage that is supplied to a
utility power grid interface transformer. A primary-side common
node of the Utility Power grid interface transformer is connected
to Earth ground and forms a portion of the hybrid grounding
circuit. Each inverter has 1) its own set of isolation contacts to
connect as well as isolate this particular inverter from the
utility grid interface transformer, and 2) control components in
the ground fault monitoring circuit for controlling operation of
the isolation contacts based off a presence of the ground fault
detected by the ground fault monitor circuit for that inverter.
Each inverter has an input circuit that temporarily grounds the
inverter and its CPV modules. The inverter circuit receives a DC
voltage supplied from its own set of ungrounded CPV modules.
FIG. 1 illustrates a diagram of an embodiment of an ungrounded
photovoltaic system feeding a hybrid grounding circuit as well as a
ground fault monitoring circuit for inverter circuitry with
switching devices that generate three-phase Alternating Current
(AC) voltage supplied to a utility power grid interface
transformer.
This is a "grounded" system using transformer-less inverter
circuits 102-105, typically generating 480 V 3-phase power, in
which 1) the primary side common node of the Utility Power grid
interface transformer 106 is connected to Earth ground during the
operation of the inverter in this hybrid grounding circuit and 2)
referenced to Earth ground by a switching device, such as a first
grounding switching device 108, in the input circuit of the
inverter prior to the bridge arranged switching devices of the
inverter producing three phase AC power. Each inverter circuit
102-105 receives a DC voltage supplied from its own set of
ungrounded Concentrated PhotoVoltaic (CPV) modules.
A utility-scale photovoltaic (PV) solar electrical power generating
system may have a large number of inverters, such as a first
through a fourth inverter circuit 102-105, that feed into a common
grid-interface transformer 106 as shown in FIG. 1. The multiple
solar arrays, each with their one or more inverter circuits,
directly couple their three phase AC output to the same utility
grid transformer 106. Thus, the inverter circuitry uses a Space
Vector Modulated bridge switches, typically generating 480 V AC
three phase power, that directly couple to the utility power grid
transformer, without connection through an isolation transformer
and then to the utility grid transformer, in where a neutral wire
of the primary side of the utility power grid interface transformer
is referenced to Earth ground.
Each inverter circuit 102-105 features series-redundant AC
disconnect contactors that disconnect the inverter from the grid
feed based on conditions sensed by the inverter controller. Thus,
each inverter has its own set of isolation contacts to connect as
well as isolate this particular inverter from the utility grid
transformer 106, control components in the ground fault monitoring
circuit, such as control logic and a set point, for controlling
operation of the isolation contacts, based off a presence of the
ground fault detected by the ground fault monitor circuit for that
inverter.
Each inverter circuit in the grounded system has its own ground
fault monitoring circuit. The ground fault monitoring circuit
detects the presence of the ground fault via a change in 1)
voltage, 2) current or 3) both and the detection method occurs via
a differential sense method. The input of each ground fault
monitoring circuit can be equipped with a residual current monitor
(RCM). The residual current monitor senses the unbalanced
("residual") current condition between the positive and negative
leads of the PV module array caused by current leakage from the
array, and signals the inverter controller to disconnect the
inverter from the grid feed if the residual current level indicates
a hazardous condition. In additional embodiments, the sensing of an
open fuse in the input can indicate the presence of a ground fault
as well.
The grounded solar electric generating system may employ additional
devices to disconnect and/or shut down individual inverters with
ground faults for safety reasons or to prevent shut down of the
whole solar system. Inverter groups (or less-commonly, individual
inverters) are interfaced to a facility bus via ground-fault
circuit interrupt (GFCI) breakers. If a ground-fault circuit
interrupt breaker senses asymmetrical power flow in the AC phases,
it disconnects the inverter group (or single inverter) from the
facility grid.
FIGS. 2a and 2b illustrate a diagram of an embodiment of a hybrid
grounding circuit and a ground fault monitoring circuit in the
inverter circuitry that generates AC voltage from the DC supplied
from the CPV solar array.
Referring to FIG. 2a, the series of CPV modules in the solar array
is arranged as two substrings that are connected to the inverter.
The inverter circuit receives a bipolar DC voltage, + and -600 VDC,
supplied from its own set of CPV modules.
When an inverter is connected to the system bus, its CPV module
array must be floating (ungrounded) to prevent a ground loop path
between the solar array and grid interface transformer primary 206.
Nevertheless, the inverter switching action maintains the midpoint
of the series-connected PV module array near ground potential.
The hybrid grounding circuit maintains the CPV array at a safe
voltage while the isolation contacts of the inverter circuit open
to disconnect from the grid. In typical systems, when an inverter
is turned off and disconnected from the grid while its CPV array
remains illuminated, then the voltages at the PV poles depend on
the ground leakage resistance balance along the array, with the
result that the voltage at one pole with respect to ground may
exceed a regulatory safety limit. However, the hybrid grounding
circuit maintains the midpoints of the DC arrays at Earth ground
prior to the isolation contacts of the inverter circuit closing and
the switching devices of the inverter producing three phase AC. The
hybrid grounding circuit includes an input DC grounding circuit 214
located in each inverter circuit that electrically couples to a
string of CPV cells from the solar array and the primary-side
common node 215 of the Utility Power grid interface transformer
connected to Earth ground. The electrical components in the input
DC grounding circuit 214 cause the DC power from that strings of
CPV cells to be connected to Earth ground when the inverter circuit
is not producing three phase AC power out. The electrical
components in the input DC grounding circuit 214 may include 1) one
or more switching devices, such as a contact, transistor, FET, or
similar device, one or more load resistors, and one or more over
current devices such as 1) fuse, 2) circuit breaker or 3) a
combination of both. The voltage from the CPV modules forming the
solar array is referenced to Earth ground via the one or more
contacts, one or more resistors, and one or more fuses when the
inverter logic is off and when powered but not producing three
phase AC voltage. The electrical components in the input DC
grounding circuit 214 may also include 1) a contact 2) switch or 3)
both to create a dynamic ground/common zero Volts DC reference
point for the bipolar DC voltage supplied to that inverter from its
set of CPV modules. The switching device may actuate, A) by
electrically opening a contact or switch, B) electrically by
closing a contact or switch, C) by the switching device starting to
conduct, and D) by any combination of the three, to create an
electrical path.
Referring to both FIGS. 2a and 2b, in an example, the `PV array DC
common reference point and grounding` control relay CRG is
de-energized while the inverter is turned off. The hybrid grounding
circuit 214 in the inverter circuit has the DC common reference
point and grounding control relay actuate one or more contacts,
such as normally open contact CRG, to create a common DC reference
point for a bipolar DC voltage input from the CPV modules of a
solar array while the inverter circuit is turned off. The DC common
reference point and grounding control relay also actuates one or
more contacts, such as normally closed contact CRG, to create
connect the array to Earth ground while the inverter circuit is
turned off, and also while the inverter logic is powered but the
inverter circuit is disconnected from the grid and not operating.
For clarity, the PV array midpoints connect to Earth ground also
via the resistors and fuses in the electrical path with the
normally closed contact CRG as shown. In these conditions, the
inverter bridge switches are open, resulting in an electrical open
circuit at each CPV array pole, and consequently, no current flows
in the CPV solar array even if well-illuminated by sunlight because
a complete ground path cannot be established. However, the
midpoints of the ungrounded CPV array are maintained at Earth
ground potential.
When a ground fault on the arrays is not present, a complete
electrical path from the positive or negative voltage of the CPV
modules to Earth ground and back up to CPV modules cannot be
established. The ground fault monitoring circuit 220 detects when a
ground leak occurs in the CPV modules. For example, a ground
current will flow through the CPV modules, ground fault resistance
RLEAK, load resistor R1, fuse F1, and the normally closed contact
CRG completing the ground path between the normally ungrounded CPV
modules and Earth ground. With the ground fault current flowing,
the ground fault monitoring circuit 220 can detect the presence of
the ground fault on the ungrounded CPV array. In addition, should
the fault current exceed the fuse rating it will open and interrupt
the fault current. The voltage will now be sensed on the open sense
input legs of a voltage differential sense circuit (see FIG. 3 for
an example differential sense circuit).
When the `PV array DC common reference point and grounding` control
relay CRG is energized, the normally open contact connects the
substrings together while the normally closed contact releases the
grounded node. The switching device in the input of the inverter
circuit, this normally open contact, creates the DC common
reference point for the positive VDC and the negative VDC inputs
from the PV array. The ground fault monitoring circuit 220 can
localize the ground fault to a specific set of CPV modules feeding
a specific inverter circuit by when the detected ground fault
voltage has a negative voltage component, then the ground fault is
coming from the set of CPV modules supplying the negative DC
voltage; and likewise when the detected ground fault voltage has a
positive voltage component, then the ground fault is coming from
the set of CPV modules supplying the positive DC voltage.
In addition, the DC sensitive residual current monitor at the
inverter input can detect PV array ground leakage prior to the
inverter being connected to the grid feed, which may prevents a
safety hazard, equipment damage, or the disconnection of an
inverter group by its ground-fault circuit interrupt senses
breaker. The latter outcome if not prevented could cause both loss
of revenue while the inverter group is off-line, and potentially
large maintenance costs to locate the ground fault to a particular
PV array. The residual current monitor (RCM) monitors residual
currents coming from the PV arrays and issues a signal when these
currents exceed a defined value. The set point for the DC fault
current of the residual current monitor is set so DC current
injection is not considered a ground fault current.
The ground fault detection circuit 220 senses a CPV array ground
fault before the inverter is put into operation so that the
inverter can be left disconnected. The ground fault detection
circuit also disconnects a single CPV array and its inverter with a
ground fault without shutting down the entire system supplying
power to the grid interface transformer. The signal is sent out
from the ground fault detection circuit 220 to open only this
inverter circuit isolation contacts CR1 and CR2.
The ground fault voltage and current detection occurs via a
differential sense and fuse method. The ground fault monitoring
circuit 220 may contain a DC imbalance sensing circuit to assist in
fault protection in determining which set of CPV modules supplying
power to the common utility grid transformer has a ground fault
both 1) prior to inverter start up and 2) during the inverter
circuit operation while producing the three phase AC voltage.
The voltage differential sense circuit to sense an open fuse is
shown in FIG. 3.
Referring to FIGS. 2a and 2b, in an embodiment of the ground fault
detection circuit 220, should a ground leak, symbolized by RLEAK,
occur somewhere along the substring, a ground current will flow
through RLEAK, load resistor R1, and fuse F1 according to the
number of photocells between RLEAK and load resistor R1, their I-V
(current-voltage) characteristics, and the intensity of solar
illumination on these cells. Should the fault current exceed the
fuse rating it will open and interrupt the fault current. The
voltage will now be sensed on the open sense input legs. The same
logic applies to the lower substring.
The ground fault monitoring circuit 220 detects when the inverter
is scheduled to put be on-line, the inverter controller
interrogates the open fuse sense lines and when the inverter
controller senses any voltage between the two open fuse sense
lines, a fuse has opened, indicating a ground fault in the CPV
solar arrays, and the inverter controller keeps the inverter
circuit off-line by keeping the isolation contacts open. The
controller keeps the inverter off-line with the CR1 and CR2 relays
de-energized. When the ground fault monitoring circuit detects no
fuse failure, the inverter controller puts the inverter on-line
with the grid by closing the isolation contacts, and also energizes
the `PV array DC common reference point and ground` control relay
CRG to interconnect and float the CPV array DC voltage midpoints.
Now, the system ground is now established by the system transformer
common being connected to Earth ground, and the residual current
monitor is now active and responsive to future CPV array ground
leaks. When a ground occurs in a CPV array, the ground fault is
sensed by the associated residual current monitor, and the solar
array with the ground fault and its inverter circuit are isolated
by actuation of the isolation contacts from the utility grid
transformer but the transformer continues to receiver AC voltage
from the other inverter circuits coupling to that transformer and
continues to produce power.
FIG. 4 illustrates a diagram of an embodiment of two inverters from
the same array having their own isolation contacts and supplying
power to a common grid transformer.
Each inverter circuit 402, 403 receives it portion of the bipolar
DC voltage supplied from its own set of CPV modules. The first
inverter 402 receives a positive DC voltage, such as +600 VDC, and
the second inverter 403 receives a -600 VDC.
The ground fault voltage and current detection still occurs via a
differential sense method. Each grounded inverter system has its
own ground fault monitoring circuit such as a first ground fault
monitoring circuit 420.
The Ground fault monitoring circuit is configured to detect PV
array ground leakage before the inverter is connected to the grid
feed, which prevents disconnecting the entire system due to the
grounded PV array. The hybrid grounding circuit in the inverter
circuit uses relays to maintain the set of CPV modules from the
solar array at a safe voltage while the inverter circuit is
disconnected from the grid by connecting a DC output of the set of
CPV modules feeding that inverter through a switching device to an
Earth ground. The PV array ground faults that occur while the
inverter is off or are present when the inverter is scheduled to be
put on-line are sensed prior to inverter operation, and the
inverter is kept off-line. The ground fault monitoring circuit is
configured to detect PV array ground leakage before the isolation
contacts of the inverter circuit connect to the utility power grid
interface transformer, which prevents disconnecting all of the
inverter circuits in the entire system due to a ground fault on a
single set of CPV modules.
The isolation contacts and control components of the ground fault
monitoring circuit are configured to prevent the 1) disconnection
of the entire solar power generating system or 2) disconnection of
an inverter group from the utility power grid interface transformer
due to a ground fault occurring in an individual inverter circuit
or its associated CPV modules, by a localization of the ground
fault to 1) a specific inverter circuit from an inverter group
coupling to the utility grid power transformer or 2) even more
specifically, a specific set of CPV modules feeding a specific
inverter circuit, which also reduce corrective maintenance
costs.
Each inverter has its own set of isolation contacts to isolate this
particular inverter from the utility grid transformer, a control
components such as relay and set point for controlling operation of
the isolation contact, based off the fault current detected by the
ground fault monitor for that inverter.
Like FIG. 1, multiple solar arrays, now each with their two or more
inverters, directly couple their three phase AC output to the same
utility grid transformer.
The hybrid grounding circuit when in operation producing AC, the
inverter circuit gets referenced to the neutral potential on the
primary side common node of the Utility Power grid interface
transformer, and when the inverter circuit is not in operation,
then an input circuit of the inverter circuit connects through a
switching device, such as a contact, to Earth ground. A switching
device may actuate, by electrically opening a contact or switch,
closing a contact or switch, by the first switching device starting
to conduct, and any combination of the three, to create an
electrical path.
The Ground fault monitoring circuit assists in fault protection in
determining which solar array supplying power to the common utility
grid transformer has a ground fault prior to inverter start up and
during the inverter operation.
In such a system, the PV modules and associated DC wiring are not
isolated from the inverter output and therefore carry hazardous
voltages. The ground fault monitoring circuit is configured to
detect the presence of the ground fault in the ungrounded CPV
modules that supply Direct Current (DC) power to grounded inverter
circuit.
A Method of Solar Cell Substring Grounding to Manage Inverter Input
Voltage for PV Applications
FIG. 6 illustrates a diagram of an embodiment of a string of CPV
modules and their CPV cells supplying power to an inverter circuit.
The most economical and reliable means of converting the DC output
of a series-wired string of solar cells is to operate the string
into a single-stage DC-AC inverter. However, a string that conforms
to the safety code for maximum voltage may not provide sufficient
voltage for single-stage inversion to AC grid power. In some
embodiments, this technique allows the use of longer,
higher-voltage strings without violating safety requirements so
that single-stage inversion can be used with a wider variety of
solar cells and AC grid voltages.
Briefly, in order to obtain the maximum power converter input
voltage within safety limits, a series-string of solar cells is
typically grounded at its midpoint (FIG. 1) so that no point of the
string exceeds +/-600 Vdc (US) or +/-1000 Vdc (EU) with respect to
utility ground. This creates a bipolar string. The string voltage
must conform to these limits under all conditions. Table 1 shows
Vmp (hot) for Si and MJ GaAs cell strings whose Voc (cold) are at
the US and EU safety limits. The table also shows the minimum
string voltage required for several types of single-stage pulse
width modulated (PWM) inverters to operate into both US and EU
power grids.
The solar cell string voltage behavior is as follows:
The highest voltage occurs at the extreme low temperature that the
plant will experience with the string unloaded (inverter off), such
that each cell is producing its open-circuit voltage, Voc. Call the
resulting string voltage Voc (cold).
The lowest voltage occurs at the extreme high cell operating
temperature when the string is loaded for maximum power extraction.
Here, each cell is operating near its maximum power voltage, Vmp.
Call the resulting string voltage Vmp (hot).
Both Voc and Vmp for a string decrease when various cells are not
illuminated (panel shading) or have failed in fail-open mode
(bypassed by diode) or in fail-short mode.
Typical silicon solar cells have relatively low fill factor
(essentially the ratio of Vmp to Voc) compared with multi-junction
(MJ) GaAs cells used in CPV applications. The difference in fill
factor has significant implications. Table 1 shows Vmp (hot) for Si
and MJ GaAs cell strings whose Voc (cold) are at the US and EU
safety limits. FIG. 8 illustrates a table of an embodiment of
available verses required DC input voltage from a string of CPV
modules.
This system uses space vector modulation (SVM) inversion since it
requires some 15% lower input voltage to operate into a given grid
voltage. For US applications, a single-stage SVM inverter requires
more input voltage than is available from a safety-compliant
silicon cell string, but has a 905-779=126 Vdc margin when powered
by a safety-compliant MJ GaAs string. However, this margin is
eroded as cells in the string are shaded or fail. A bipolar string
may comprises 360 MJ GaAs cells from 15 panels (modules), and the
margin is erased if more than about 1.5 panels are shaded.
Therefore, a means of obtaining higher solar cell string operating
voltages will allow a plant to produce power under adverse
illumination conditions and with more tolerance for solar cell
failures.
FIG. 5 illustrates a diagram of an embodiment of a method of solar
cell string voltage management, used in conjunction with a
single-stage SVM inverter. In an embodiment, the three-substring
implementation of a grounded substring is as follows.
The solar cell string is divided into a set of substrings. The ends
of one or more substrings are connected to utility ground via
normally-closed relay contacts when the inverter is not running. As
illustrated here, there are three substrings labeled upper, middle,
and lower, of which the middle substring is grounded via relay
contacts. In the implementation proposed, the upper and lower
substrings each comprise six modules (144 MJ GaAs cells in series),
while the middle substring comprises three modules (72 such cells
in series). In this arrangement, Voc (cold) for the upper and lower
substrings is about 480 Vdc each, which is well within the +/-600
Vdc safety limit.
Alternatively, the string could be assembled from a larger number
of cells for an overall Voc (cold) in excess of the safety limit
(e.g. end-to-end voltage >1200 Vdc) and then arranged into
various length substrings, one or more of which is grounded via
normally-closed relay contacts such that no part of the string
exceeds +/-600 Vdc with respect to ground. This would be
advantageous for silicon solar cells or other types of solar cells
having low fill factor. Say, for example, it is desired to operate
a SVM inverter from a string of silicon cells such that Vmp (hot)
is 900 Vdc, enabling conversion to 480 V grid power with good
margin. Scaling from Table 1, the string would have a Voc (cold) of
about 1200.times.(900/593)=1821 Vdc. To conform to the safety
limit, the string could be divided into substrings such that the
upper and lower substrings have a Voc (cold) of say 580 Vdc and the
middle substring has a Voc (cold) of 661 Vdc.
How the Technique Works
In some embodiments, when the inverter turns on, it loads the
ungrounded substrings such that their voltage is pulled down to or
below the anticipated Vmp for those substrings. The inverter
controller then energizes the relay that ungrounds the ends of the
middle substring, thereby presenting the full, loaded string
voltage to the inverter input. The inverter then goes into
regulated operation with the string at Vmp, and actively manages
one sting endpoint voltage with respect to utility ground to
maintain compliance with the +/-600 V safety limit over the length
of the string.
The inverter controls switching such that the amount of current
sourced to the grid loads the solar cell string at its maximum
power point (MPP) over conforming grid voltages and also over
varying solar cell illumination levels. When the power available
from the solar cells falls below a specified minimum, the inverter
re-grounds the middle string, turns off the power path, and
monitors the remainder of the string for the resumption of adequate
power. Also, if the inverter senses that the voltage at either of
its inputs exceeds the safety limit, it re-grounds the middle
string and turns off.
Advantages include one or more of the following:
Increased tolerance to poor illumination conditions and solar cell
failures. For a given grid power form and solar cell type, the
technique allows greater operating input voltage, Vmp, than would
otherwise be available from the solar cell string. This improves
system tolerance to panel shading, low early morning and late
evening illumination levels, and solar cell failures.
Standardized inverter design accommodates various solar cell types
and grid power forms. The technique allows an inverter of some
standardized design to be used with a wide variety of solar cells
having different fill factors, while operating into a wider range
of grid power voltages. The ability to standardize an inverter
design for a wide range of applications reduces inverter
development cost and risk, and reduces recurring manufacturing
cost.
Use of lower cost inverter parts with lower voltage rating. The
ability to reduce the string Voc to which the inverter input is
exposed allows the inverter to be implemented with lower
voltage-rated parts. For example, the substring grounding scheme
described above applies only 12/15 of full string Voc (cold) to the
inverter at startup, or some 960 V. When loaded, the entire string
will present a maximum voltage, Vmp (cold) of some 996 V. This
allows the use of 1200 V switching devices that are derated to 1000
V for improved reliability margin. Minimal impact on inverter
complexity. Since the basic single-stage SVM inverter is designed
to operate a relay to unground the midpoint of the solar cell
string, implementing the substring grounding method of the
disclosed technique has minimal impact on inverter design and
negligible impact on recurring cost. As stated above, the technique
applies to the grounding of a single substring or to multiple
substrings of solar cells.
FIG. 7 illustrates a diagram of an embodiment of the physical and
electrical arrangement of modules in a representative tracker unit.
Here, there are 24 power units per module, eight modules per
paddle, two paddles per tilt axis, and four
independently-controlled tilt axes per common roll axis. As
discussed, the bi-polar voltage from the set of paddles may be, for
example, a +600 VDC and a -600 VDC making a 1200 VDC output coming
from the 16 PV modules. The 16 PV module array may be a string/row
of PV cells arranged in an electrically series arrangement of two
300 VDC panels adding together to make the +600 VDC, along with two
300 VDC panels adding together to make the -600 VDC.
Although the foregoing embodiments have been described in some
detail for purposes of clarity of understanding, the invention is
not limited to the details provided. The Solar array may be
organized into one or more paddle pairs. CPV modules on the West
side and East side may supply different amounts of voltage or
current. Functionality of circuit blocks may be implemented in
hardware logic, active components including capacitors and
inductors, resistors, and other similar electrical components.
There are many alternative ways of implementing the invention. The
disclosed embodiments are illustrative and not restrictive.
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